feat(zgw): real per-request identity seam + citizen-scoping (WP-53)

Replaces the hardcoded DocumentStore.DemoOwner and the static ZgwOptions
UserId/UserRepresentation with one per-request CallerIdentity, resolved by a
pluggable IIdentityProvider (StubIdentityProvider reads X-Role/X-Subject
today; a real OIDC/DigiD provider swaps in without touching any consumer).

- Domain/Authorization/{CallerIdentity,IIdentityProvider,StubIdentityProvider}.cs
  + a resolution middleware in Program.cs, right after correlation-id.
- Authz.ResolvePrincipal(ctx) keeps its signature (now reads ctx.Caller().Role),
  so its ~15 call sites needed no changes.
- Every endpoint that passed DocumentStore.DemoOwner to a store now passes
  ctx.Caller().Bsn.
- ZgwTokenProvider gains Mint(CallerIdentity) alongside the original Mint()
  (kept for calls not tied to one citizen); ZgwHttpClient threads an optional
  caller through to pick the right overload.
- IZaakSource gains ListMyCases(caller, now) — the citizen-scoped read
  OpenZaakZaakSource backs with ZGW's rol__...__inpBsn filter. GET /applications
  now routes through it instead of ApplicationStore directly, closing the last
  "reads a static store" gap for a citizen-facing endpoint.

Backend 159/159 tests (+8, incl. an HTTP-level two-identity scoping proof),
npm run ci green, no api-client drift.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/project/backlog/README.md

@@ -103,7 +103,7 @@ for its existing violations, so every WP ends green.
 | [WP-50](WP-50-openzaak-create-zaak.md)                 | OpenZaak create-zaak (first write slice)                                           | 9 · OpenZaak/ZGW            | done   |
 | [WP-51](WP-51-openzaak-documenten.md)                  | OpenZaak Documenten (DRC) upload + zaak link                                       | 9 · OpenZaak/ZGW            | done   |
 | [WP-52](WP-52-openzaak-notificaties.md)                | OpenZaak Notificaties (NRC) live status via webhook                                | 9 · OpenZaak/ZGW            | done   |
-| [WP-53](WP-53-inbound-identity-and-citizen-scoping.md) | Inbound identity seam + citizen-scoping (per-request BSN, ZGW audit claims)        | 9 · OpenZaak/ZGW            | todo   |
+| [WP-53](WP-53-inbound-identity-and-citizen-scoping.md) | Inbound identity seam + citizen-scoping (per-request BSN, ZGW audit claims)        | 9 · OpenZaak/ZGW            | done   |
 | [WP-54](WP-54-openzaak-integration-harness.md)         | Docker OpenZaak integration-test harness (opt-in, live round-trip)                 | 9 · OpenZaak/ZGW            | todo   |
 
 Sequencing dependencies (stated in the WPs too): 01 before 10–15 (axe covers story churn);